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We report on an experimental and theoretical study of a large-aperture Ti:Sapphire (Ti:S) amplifier pumped with a novel temporal dual-pulse scheme to suppress the parasitic lasing (PL) and transverse amplified spontaneous emission (TASE) for high-energy chirped-pulse amplification (CPA). The pump energy distribution was optimized and the time delay between each pump pulse was controlled precisely. Both the numerical and experimental results confirm that the temporal dual-pulse pump technique can effectively suppress PL and TASE. The maximum output energy of 202.8 J was obtained from the final 150-mm-diameter Ti:S booster amplifier with a pump energy of 320.0 J, corresponding to a conversion efficiency of 49.3%. The compressed pulse duration of 24.0 fs was measured with a throughput efficiency of 64%, leading to a peak power of 5.4 PW. This novel temporal dual-pulse pump technique has potential applications in a 10 PW CPA laser system.
© 2017 Optical Society of America
1. Introduction
High-power femtosecond lasers have undoubtedly revolutionized the scientific world. Due to their ability to generate ultrahigh intensities within extremely short time intervals, petawatt femtosecond-class lasers have been developed for many specific research activities, including studying laser-matter interaction at ultrahigh intensities [1], accelerating charged particles (electrons and protons) to relativistic velocity [2] and generating coherent or incoherent high energy radiation [3]. Until now, using the chirped-pulse amplification (CPA) technique and its analogue, the optical parametric chirped pulse amplification (OPCPA) technique, petawatt femtosecond-class lasers are routinely realized in many laboratories and companies globally [4–9]. There are several promising projects that are in either the preparatory design phase or the onset of implementation intended to reach 10 PW-level outputs, such as ELI [10], Vulcan-10PW [11], SIOM-10PW [12, 13], and PEARL-10PW [5]. Compared to the OPCPA technique, the CPA technique, particularly using Ti:sapphire (Ti:S) CPA systems has a number of important advantages such as higher stability and efficiency as well as lower requirements of the pump laser. These features make the CPA technique the workhorse in most petawatt class laser systems with outputs of >1PW, and the CPA technique is thought to be on the verge of maturing to achieve outputs of several petawatts and even 10-PW laser pulse outputs. As a single-beamline prototype for the ELI project, the APOLLON 10-PW facility in progress aims to generate a pulse of 150 J/15 fs with a power of 10 PW based on the Ti:S CPA technique [14, 15]. A new project, the Shanghai Super-intense Ultrafast Laser Facility (SULF), which plans to launch a pulse of 280 J/28 fs with 10 PW output in 2018, is also designed using the Ti:S CPA technique.
To achieve a peak power of several petawatts or even 10 PW of output from a Ti:S CPA laser system, crystals with a diameter in the range of 150-220 mm are necessary for the final amplification stage. Nevertheless, the main limitation that arises when designing such large- aperture, high-gain amplifiers is restrictions on the pump energy storage and signal energy extraction imposed by parasitic lasing (PL) [16, 17] and transverse amplified spontaneous emission (TASE) [6, 18]. PL is due to the formation of a laser cavity through Fresnel reflections at the material interfaces of the gain medium. TASE, whose losses are associated with one pass ASE, may place an even stronger restriction on output power than PL, as it definitively increases with increasing the crystal aperture size, limiting the maximum stored energy. Currently, techniques for suppressing PL and/or TASE involve two methods. One way is to decrease the Fresnel reflection from the crystal perimeter by applying a layer of index-matched thermoplastic polymer or flowing an index matching liquid around the cylindrical surface. We describe this method as a passive technique. A promising liquid for this is a di-iodomethane derivative (Series M, nD1.78 from Cargille Laboratories), which has an index of refraction close to 1.76, the refractive index of Ti:S crystals. However, the difficulty in introducing exact index matching in whole gain bandwidth make using passive techniques alone a failure when multi-petawatt pulse output is desired. In such case, an active technique is needed. We describe an active technique as that which involves controlling the transverse gain during the pumping process, which is otherwise known as the extraction during pumping (EDP) [19] technique, credited to Vladimir Chvykov. In this method, the temporal delay between the signal pulse and pump pulses is carefully controlled, only part of the pump energy is stored in the gain medium before the arrival of the input signal, after which a large part of the energy stored in the crystal is quickly transferred to the signal pulse as it is amplified, resulting in the transverse gain remaining low. The remaining pump pulses keep pumping for the next pass of the amplified signal pulse. By combining passive techniques with active techniques, an output of 192.3J centered at 800 nm wavelength was obtained in our recent research on a Ti:S CPA laser system [20]. However, studies on finding new methods for suppressing PL and TASE are necessary to ensure a 10-PW level output. A new scheme is proposed by using a temporal dual-pulse pump [21]. The suggested dual-pulse pump has a complex temporal profile, a relatively short and high-energy first pulse and a relatively long and low-energy second pulse. Though multi-pulse pump experiments have already been reported, in previous studies, all pump pulses are temporally superimposed or the overall pulse duration of the multi-pump beam is adjusted to be minimize [7, 22, 23], in such case, multiple low-energy pump pulse are made equivalent to a single energetic pump pulse.
In this study, we perform a complete numerical and experimental study of the dual-pulse pumped Ti:S CPA technique. The pulse interval between each pump pulse is deliberately increased so that the pumping process is maintained throughout the entire amplification stage. Rather than the relatively complex temporal profile proposed in [21], here, each pump pulse temporal profile is identical, while the energy of each pump pulse is different, corresponding to the practical situation where one high-energy pump pulse is divided into multiple, individual pulses using beamsplitters. In section 2, We show the advantages of the temporal dual-pulse pump scheme over the traditional method where one long pump pulse is used by means of a numerical simulation. The energy distribution of each pump pulse is optimized. We then carry out a temporal dual-pulse pumped Ti:S CPA experimental study under the guidance of this simulation, as shown in section 3. The laser system produced an uncompressed output pulse with an energy of 202.8J and demonstrated compression to 24.0fs, with a measured 64.0% throughput efficiency of the compressor, indicating a peak power of 5.4PW. To the best of our knowledge, this is the highest energy and peak-power output ever reported for a petawatt femtosecond-class laser system.
2. Numerical Simulation
In this section, we show the advantages of the temporal dual-pulse pump scheme over the traditional one pulse pumped scheme by numerical simulation. The main idea of the temporal dual temporal pulse pumped scheme is to optimize the pump energy distribution in each stage of the final booster multi-pass amplification.
2.1 Traditional one pulse pumped scheme
According to the numerical model described in [24], we first study the traditional case where the Ti:S crystal is double-end pumped, and there is only one pump pulse at each end. The input signal is 45 J with a 115-mm-diameter. Each end pump pulse has an energy of 160 J with a 120-mm-diameter and the same super-Gaussian temporal profile with a 16 ns pulse width. The two end pump pulses arrive at the Ti:S crystal faces simultaneously while the time delay between the signal pulse and pump pulses can be adjusted. We compare the maximum surface transverse gain before the signal pass of the Ti:S crystal in each stage of the three-pass amplification using two different doped Ti:S crystals. One is a lightly doped 150 mm × 46.7 mm Ti:S crystal and the other is a heavily doped 150 mm × 35 mm Ti:S crystal. The small pump light absorption coefficient is kept as a constant 92.6% in both cases. As the signal pulse has a lower energy and consumes less inversion population at the crystal entrance face than at the exit face, it will accumulate more inversion population before the signal makes its next pass. The entrance face in one stage will become the exit face in the next stage, so that the exit face has the maximum surface transverse gain in each stage. The pump pulses are completely absorbed before the third pass for high transfer efficiency. The numerical results are shown in Fig. 1. TGmax1, TGmax2 and TGmax3 are the maximum transverse gains of the two crystal faces before the signal passes through the Ti:S crystal in each stage of the three-pass amplification. Eamp1, Eamp2 and Eamp3 are the amplified signal energies after signal passes through the Ti:S crystal in each stage of the three-pass amplification. Despite the much larger value in the case of the heavily doped Ti:S crystal as compared to the light doped Ti:S crystal, the transverse gain evolution versus time delay is similar in both cases. When the time delay between the signal pulse and pump pulses is small, the largest transverse gain is obtained before the second time signal passes through the Ti:S crystal and the loss due to PL and TASE is serious at the second stage. As the time delay increases, TGmax1 catches up with the TGmax2 and finally become the same as the TGmax2 at some critical time delay, which corresponds to the optimized time delay when the traditional one pump pulse is used. The maximum transverse gain is 342 and 2404 for the lightly doped and heavily doped Ti:S cases, respectively, under the optimized time delay condition. When the time delay continues to increase, TGmax1 peaks, indicating that the largest transverse gain is obtained before the first time signal passes through the Ti:S crystal and the loss related to PL and TASE is serious at the first stage. TGmax3 is close to zero at any given time delay, indicating an unreasonable pump energy distribution, as the processing load is concentrated in the first two stages while it is overly relaxed in the third stage.
Fig. 1 The maximum transverse gains (red curve) of the two crystal faces before the signal passes through the Ti:S crystal and the amplified signal energy (blue curve) after signal passes through the Ti:S crystal in each stage of the three-pass amplification under different signal-pump time delay condition. (a) corresponds to the lightly doped Ti:S case, (b) corresponds to the heavily doped Ti:S case.
2.2Temporal dual-pulse pumped scheme
The schematic of the temporal dual-pulse pump scheme is shown in Fig. 2. One energetic pump pulse Ep is divided into two same-energy pulses, Ep1 and Ep2, by means of beamsplitter BS0 with a 1:1 splitting ratio. Ep1 and Ep2 are each further decomposed into two individual pulses by beamsplitters BS1 and BS2. The Ti:S crystal is double-end pumped, and there are two pulses at each end. In the temporal dual-pulse pump scheme, the pump pulses have a relatively short pulse-width about 8.0 ns. We adjust the optical path difference to set an appropriate time delay between each of the individual pump pulses. We assume Ep1-1 is absorbed in the first stage, Ep1-2 and Ep2-1 are absorbed in the second stage, and Ep2-2 is absorbed in the third stage. The pump energy of Ep1-1 should be relatively large to ensure an adequate gain amplification in the first stage. However, as the signal energy is low before the first time pass through the Ti:S crystal, after the first pass amplification, there remains large degree of inversion population. The pump pulses of Ep1-2 and Ep2-1 should refuel the consumed stored energy in the first stage while avoiding overdriving the gain medium. As the energy of the signal pulse before the second pass through the Ti:S crystal is relatively large, a large amount of inversion population is converted into signal energy. The pump pulse of Ep2-2 then continues pumping the gain medium to prepare for the third-pass amplification stage. The splitting ratios of BS1 and BS2 are crucial, as they decide the pump energy distribution throughout the entire amplification stage. In the temporal dual-pulse scheme, the splitting ratios of BS1 and BS2 are optimized when the values of TGmax1, TGmax2 and TGmax3 are identical, which means the processing load is shared throughout the whole three-pass amplification process.
The numerically calculated optimized spitting ratios of BS1 and BS2 under different pump pulse energies are shown in Figs. 3(a) and 3(b). Figure 3(a) corresponds to the lightly doped Ti:S crystal case, while Fig. 3(b) corresponds to the heavily doped Ti:S crystal case. In both cases, the reflectivities of BS1 and BS2 are about 34.5% and 55.5%, and they increase slightly as the pump pulse energy increases. The relatively large energy of Ep1-1 and Ep2-2 coincides with our previous analysis. The optimized reflectivities of BS1 and BS2 are dependent on the pump energy, while in practice, it may be difficult to get beamsplitters exactly equal to the optimized ones. We then numerically calculate the transverse gain in the temporal dual-pulse pumped scheme when the reflectivity of BS1 and BS2 deviates from the ideal case. The results are shown in the Figs. 3(c) and 3(d). In this simulation, the reflectivities of BS1 and BS2 are 30% and 55% respectively, corresponding to the real experimentally tested condition (indicated in the next section). Under a relatively low pump energy, TGmax1, TGmax2 and TGmax3 is close to TGmax, which is the transverse gain in the optimized condition. As the pump energy increase, TGmax1 exceeds TGmax further. However, in the temporal dual-pulse pumped scheme, the values of TGmax1, TGmax2 and TGmax3 are comparable, indicating that the processing load is shared throughout the entire amplification stage. When the total pump energy is 320 J, the maximum transverse gain in the whole amplification stage is 195 and 1136 for the lightly doped and heavily doped Ti:S cases respectively, lower than that in the traditional one pulse pumped scheme, indicating that the effect of PL and TASE is decreased.
Fig. 3 The optimized reflectivities of BS1 (red curve) and BS2 (blue curve) with respect to pump energy in lighted doped Ti:S case (a) and heavily doped Ti:S case (b). The maximum transverse gains (red curve) of the two crystal faces before the signal passes through the Ti:S crystal and the amplified signal energy (blue curve) after signal passes through the Ti:S crystal in each stage of the three-pass amplification under different pump energy. (c) corresponds to the lightly doped Ti:S case, (d) corresponds to the heavily doped Ti:S case.
3. Experiment
3.1 Experimental setup
The overall layout of the CPA experimental setup that we have implemented to test our proposed temporal dual-pulse pumped scheme is presented in Fig. 4(a). The broadband seed pulse output from a robust, high-contrast front-end is expanded by means of an all-reflective Offner type stretcher to a width of about 2 ns, and amplified to 2 mJ by a high stability regenerative amplifier (RA). A spectral shaping filter is inserted into the RA to suppress spectral gain narrowing and control the spectral shape. The signal beam is expanded and sent to three multi-pass Ti:S amplifiers, with an energy output of 7 J. It is then reshaped by a soft edge aperture with a 5.8 J energy output, and sent to the following power and booster amplifiers that are pumped by two frequency-doubled home-made Nd:glass lasers in single shot mode. The three-pass 80-mm-diameter Ti:S amplifier is double-end pumped by two 18 ns, 50 J pulses, each having a diameter of 70 mm. The signal pulse has a 45 J energy output and a beam diameter of 63 mm in diameter. The amplified signal pulse is further enlarged to 115 mm in diameter prior to the final injection into a three-pass 150-mm-diameter Ti:S amplifier. The pump laser for the booster amplifier is originally designed to pump an OPCPA laser system. The pulse width can be adjusted between 1 and 10 ns. A temporal shaper is used in the front end to preemptively compensate for pulse distortion in the following Nd:glass amplifiers. The pump laser system can emit a high quality pulse having a nearly rectangular temporal shape. The maximum output energy from the pump laser is 800 J at 527 nm. When this Nd:glass laser system is used as a pump source for the 150-mm-diameter Ti:S amplifier, the pulse width is set to be 8 ns, and the pulse energy at 527 nm is keep below 320 J for safe operation.
Fig. 4 (a) Overall layout of the CPA experimental setup. (b) Schematic for the temporal dual-pulse pumped Ti:S amplifier experimental setup. (c) Signal-pump time-delay control in the experiment.
The experimental setup of the temporal dual-pulse pumped Ti:S amplifier is shown in Fig. 4(b). The output of the pump laser system is down-collimated to a diameter of 120 mm. This energetic pump pulse is divided into two pulses with the same energies by beamsplitter BS0. One pump pulse is image-relayed to the other end of the Ti:S crystal. These two pump pulses are then further divided into two individual pulses respectively by beamsplitters BS1 and BS2. There are two pump pulses at either end of the Ti:S crystal. The reflectivities of BS0, BS1 and BS2 are 50%, 30% and 55% respectively. The signal-pump time delays are shown in the Fig. 4(c). The zero time is defined as the moment when the front edge of the pump pulse arrives at the surface of the Ti:sapphire crystal. In practice, the time delays between the signal and pump pulses were electronically synchronized using a Master Clock (THALES, Inc.) circuit, while the relative time delays between each pump pulses were controlled for by carefully adjusting the optical path difference. The time interval between the two signal passes is set to be 15 ns and 14.4 ns in the practical three-pass 150-mm-diameter Ti:S amplifier. The pulse interval between each of the pump pulses is deliberately increased to about 8 ns so that the pumping process is maintained throughout the entire amplification stage. The time delay between the input signal and the first pump pulse is set to be about 6 ns in practice. It was found that, unlike in our previous experiment when one pump pulse is used, in the temporal dual-pulse pumped scheme, the output of the 150-mm-diameter Ti:S amplifier is not as sensitive to the signal-pump time delays, as the pump energy distribution is constrained mainly by means of the beamsplitters.
3.2 Results and discussions
Based on the above temporal dual-pulse pumped technique as well as using the Cargille Series M refractive index liquid doped with an absorber (IR 140) as the cladding material, we measured the output energy and conversion efficiency with respect to the pump energy. The input signal energy was 45 J, as output from the 80-mm-diameter Ti:S power amplifier. The measured experimental data and the numerical simulation results are shown in Fig. 5. The experimental results confirmed that the temporal dual-pulse pumped technique can effectively suppress the PL and TASE. The output signal energies and conversion efficiencies increase with pump energy, with a maximum amplified output energy of 202.8 J been obtained at a pump energy of 320 J, which corresponds to a conversion efficiency of 49.3%. The results of the numerical calculation coincide well with our experimental data at lower pump energies, while the difference between the experimental and simulated results becomes more obvious at higher pump energies due to the effect of PL and TASE, which were not considered in our simulation. After the final booster amplifier, the near-field of the amplified laser beam was measured, as shown in the inset of Fig. 5. The amplified signal pulse has a near flat-top spatial beam profile due to gain saturation, while some intensity modulations occur under the influence of the optical inhomogeneity of the 150-mm-diameter Ti:S crystal. The relative RMS intensity modulations are 15 and 14% for the horizontal and vertical directions respectively.
Fig. 5 Experimental and numerical obtained amplified output energy and conversion efficiency with respect to pump energy. The inset figure shows the near field spatial profiles of the amplified laser beam after the final amplifier.
The evolution of the laser spectra across the whole CPA laser system is shown in Fig. 6(a). By adjusting the spectral filter in the regenerative amplifier (RA), the spectrum after RA (solid blue line) was shaped to be preweighted on the blue side. As shown in Fig. 6(a), gain narrowing of the amplified spectrum was well controlled, while the central wavelength was red-shifted due to gain saturation in the Ti:S amplifiers. The spectral width of the amplified output pulse from the final booster amplifier can support a 19.9 fs transform-limited pulse.
Fig. 6 (a) Spectral evolution throughout CPA laser system; (b) Measured autocorrelation trace of the compressed pulse in the PW Ti:S laser system.
For optimizing the pulse compression, part of the amplified pulse was recompressed by a grating compressor consisting of four 1480 groove/mm gold-coated holographic gratings. The autocorrelation traces of amplified pulse were measured and shown in Fig. 6(b). Assuming a deconvolution factor of 1.5, the duration of the compressed pulse was 24.0 fs. The measured transmission efficiency of the compressor was about 64%, indicating output energy of 129.8 J for a compressed pulse, corresponding to a peak power of 5.4 PW (compressed pulse energy divided by the compressed pulse width).
4. Conclusion
In conclusion, we demonstrated the advantages of the temporal dual-pulse pump scheme to suppress the PL and TASE effects in large-aperture Ti:S amplifiers. This structure can improve the controlling of gain in Ti:S temporally. Through optimizing energy distribution of several pump pulses, the consumed energy stored in the gain medium is restored to amplify the signal, while the gain can be kept below the threshold of PL and TASE. The experimental results confirmed the effectiveness of this new scheme. With 45.0 J input energy, the 150-mm-diameter Ti:S amplifier are capable of producing a 202.8 J output under a pump energy of 320.0 J. The maximum conversion efficiency from the pump energy to the output signal is 49.3%. With the 64% compressor throughput efficiency, the pulse duration was compressed to 24.0 fs. Therefore, energy of 129.8 J and peak power of 5.4 PW was obtained from this laser system, which are the highest values ever reported for a petawatt femtosecond-class laser, to the best of our knowledge. In the next step, we will use this method in the final amplifier of our 10 PW laser.
Funding
National Natural Science Foundation of China (No. 61521093, 61378030).
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